Thin films of boron deposited over a small area by vacuum evaporation have been exposed to the National Bureau of Standards thermal-neutron flux in pulse ionization chambers. Pulse ionization measurements in the past have yielded ${\mathrm{He}}^4$-to-${\mathrm{Li}}^{7*}$ pulse ratios considerably higher than the energy ratio of 1.753 from momentum conservation. Under suitable operating conditions we have obtained ratios as low as 1.778. Columnar recombination appears to cause most of this variation. The probability of the reaction ${\mathrm{B}}^{10}(n, \alpha ){\mathrm{Li}}^7$ going to the ground state was determined to be 6.52±0.05 percent.

• Received 4 November 1953

DOI:https://doi.org/10.1103/PhysRev.93.831